The regeneration niche of trees at the alpine treeline: competition and facilitation by alpine vegetation during germination and seedling establishment
Final Report Abstract
Treelines occur on mountains worldwide at similar thermal thresholds, although thermal and climatic differences are also conspicuous. The potential elevational limit for tree growth is probably set by a lack of warmth during the growing season. However, the elevational limit for tree establishment is set by quite different climatic factors and additionally depends on biotic interactions. This may explain why few alpine treelines currently reach the potential tree-growth limit and has important bearings for treeline responses to climatic changes. Being a thermal boundary, alpine treelines are generally expected to advance upslope in response to warming, but limitations at the germination and seedling stage might hamper this advance. In this project we studied how climatic and biotic factors limit seed germination and seedling performance in five important European treeline tree species: Picea abies, Pinus cembra, Pinus uncinata, Larix decidua, and Sorbus aucuparia. These were planted at treeline elevation in the French Alps and subjected to experimental treatments modifying temperature, moisture and light availability. The most important results so far can be summarized as follows: 1. Germination and germinant survival responses to moisture and temperature were highly idiosyncratic between species, showing positive and negative responses and interactions. 2. Germination and germinant survival responded similarly to moisture and temperature within species. 3. Full and lowered alpine vegetation negatively affected seedling growth in all tree species, but artificial shade did not (except in Pinus cembra). This indicates that light is not the main limitation under vegetation but that belowground competition also limits growth. 4. Full vegetation negatively affected survival and carbohydrate stores, but lowered vegetation did not, although it affected growth. This indicates that the worst seedling performance is caused by combined aboveground and belowground competition. 5. The negative effect of alpine vegetation (competition) was seasonally changed to a positive effect (facilitation) for the evergreen tree seedlings, at least in terms of carbon gain. 6. Responses were species-specific even within the functional group of evergreen needleleaved trees and within the genus Pinus, implying that models predicting treeline change should be parameterized down to the species level. Our results highlight that tree species will probably respond idiosyncratically to climatic changes and that treelines composed of these species may differ strongly in their ability to shift upwards in response to warming and generally in their response to climatic changes. As spatial patterns of establishment also depend on the species-specific environmental responses, in a next step these patterns may be used to deduce regeneration requirements and thus to help predict future treeline dynamics.
Publications
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Ecophysiology of tree regeneration at the alpine treeline. 43rd Annual Meeting of the Ecological Society of Germany, Austria and Switzerland (2013, Potsdam, Germany)
Bader M.Y. & H. Loranger
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2014. A cool experimental approach to explain elevational treelines, but can it explain them? American Journal of Botany 101: 1403-1408
Bader, M. Y., H. Loranger & G. Zotz
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Species-specific climate responses in tree regeneration at the Alpine treeline. Joint Annual Meeting of the British Ecological Society and the Société Française d’Ecologie (2014, Lille, France)
Loranger H., G. Zotz & M.Y. Bader
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Alpine treeline responses to experimental climate change: germination and seedling establishment. 7th Biennial Meeting of the International Biogeography Society (2015, Bayreuth, Germany). Abstract P2.2 in: Frontiers of Biogeography, 6, suppl. 1: 91
Bader M.Y., G. Zotz & H. Loranger
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Are temperature and moisture limiting factors for early establishment of trees at the alpine treeline? Perth III: Mountains of our Future Earth conference (2015, Perth, Scotland, UK)
Loranger, H., G Zotz & M.Y. Bader
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La dinámica de los límites arbóreos en las montañas tropicales y templadas. VIII Congreso Colombiano de Botánica (2015, Manizales, Colombia)
Bader M.Y.
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Climate-change effects on early tree establishment at the alpine treeline. Annual meeting of the AK Hochgebirge (2016, Berchtesgaden, Germany)
Bader M.Y., H. Loranger & G. Zotz
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Early establishment of trees at the alpine treeline: idiosyncratic species responses to temperature-moisture interactions. AoB PLANTS, Volume 8, 1 January 2016, plw053
Loranger, H., G. Zotz & M.Y. Bader
